This invention relates to a synchronous rectifying drive system wherein current flows through the power switching elements in the reverse direction as well as in the forward direction, and more particularly to a semiconductor circuit capable of performing switching control with low power loss.
In the case of driving an inductive load, there are two modes: the recirculation mode wherein electric energy stored in the inductive load causes electric currents to flow through the power switching elements in the reverse direction and the normal mode wherein electric currents flow through the power switching elements in the forward direction. In the synchronous rectifying drive system, the power switching elements are reversely operated, i.e. turned on in the recirculation mode wherein the reverse voltages are applied to the power switching elements, so that voltage drops across the power switching elements are suppressed as much as possible. However, if the proper instants for synchronous rectifying drive at which the switching elements are turned on are missed, the power switching elements of the upper and lower arms may turn on simultaneously. Accordingly, there arises a possibility that large current flows from the high voltage source terminal to the ground to destroy the power switching elements and sometimes the whole system. Therefore, in order to prevent such a failure, the dead time during which the power switching elements of the upper and lower arms are turned off simultaneously is made sufficiently long enough to prevent the power switching elements from being turned on simultaneously. However, if the dead time is too long, it causes an increase in power loss so that high frequency application is hardly possible. This is a drawback with this system.
JP-A-2005-333314 discloses a method wherein the output voltage of the bridge circuit is detected by a sensor so as to shorten the dead time and the synchronous rectifying drive is effectuated with the voltages applied to the power switching elements.
JP-A-2006-502690 discloses a method wherein the synchronous rectifying drive is effectuated by providing power switching elements having relatively high on-resistances (resistances in the conductive state) in parallel to the main power switching elements.